For liquid crystal display devices, various techniques for improving viewing angle characteristics are conventionally proposed. As techniques for improving viewing angle characteristics, there are known, for example, a scheme in which “one picture element is composed of a plurality of (typically, two) sub-picture elements, and liquid crystal is driven such that the luminances of the plurality of sub-picture elements differ from each other” (hereinafter, referred to as the “space-division scheme”) and a scheme in which “for display at each picture element, display with desired luminance is performed by dividing a unit period (a period for displaying a one-screen image) into a period during which display with relatively bright luminance (hereinafter, referred to as “bright display”) is performed and a period during which display with relatively dark luminance (hereinafter, referred to as “dark display”) is performed” (hereinafter, referred to as the “time-division scheme”).
FIG. 13 is a circuit diagram showing a configuration of a portion forming one picture element (hereinafter, referred to as a “picture element portion”) in a liquid crystal display device adopting the space-division scheme. As shown in FIG. 13, a picture element portion 80 is composed of two sub-picture element portions (a first sub-picture element portion PIX81 and a second sub-picture element portion PIX82). Both of the sub-picture element portions (PIX81 and PIX82) include thin-film transistors (T81 and T82) connected at their gate electrodes to a gate bus line (scanning signal line) GL and connected at their source electrodes to a source bus line (video signal line) SL; pixel electrodes (E81 and E82) connected to the drain electrodes of the thin-film transistors (T81 and T82); liquid crystal capacitances (Clc81 and Clc82) formed by the pixel electrodes (E81 and E82) and a common electrode EC; and auxiliary capacitances (Ccs81 and Ccs81) formed by the pixel electrodes (E81 and E82) and CS bus lines (auxiliary capacitance wiring lines) (CSL81 and CSL82). Note that a pixel capacitance is formed by the liquid crystal capacitance and the auxiliary capacitance, and a constant voltage VCOM is provided to the common electrode EC. In such a configuration, when the gate bus line GL is placed in a selected state, the thin-film transistors T81 and T82 are placed in an on state. Since the source electrode of the thin-film transistor T81 in the first sub-picture element portion PIX81 and the source electrode of the thin-film transistor T82 in the second sub-picture element portion PIX82 are connected to the same source bus line SL, the voltage of the pixel electrode E81 in the first sub-picture element portion PIX81 and the voltage of the pixel electrode E82 in the second sub-picture element portion PIX82 are equal to each other. Thereafter, one of the voltages on the CS bus lines CSL81 and CSL82 is allowed to increase and the other one of the voltages is allowed to decrease, by which the voltage of the pixel electrode E81 and the voltage of the pixel electrode E82 change in opposite directions. By this, the pixel electrode E81 and the pixel electrode E82 have different voltages and thus the first sub-picture element portion PIX81 and the second sub-picture element portion PIX82 have different luminances. As a result, viewing angle characteristics are improved.
In a liquid crystal display device adopting the time-division scheme, a unit period for displaying a one-screen image consists of, for example, four frames. When taking a look at the four frames, the display state of each pixel changes in the manner shown in, for example, FIG. 14. In the example shown in FIG. 14, some pixels change in their display states in the manner “dark display, dark display, bright display, and bright display” and some pixels change in their display states in the manner “bright display, bright display, dark display, and dark display”. By thus performing display with target luminance using a bright display period and a dark display period, viewing angle characteristics are improved. Note that when the display state changes in the unit of picture elements instead of the unit of pixels, too, viewing angle characteristics are improved in the same manner.
In a liquid crystal display device adopting the space-division scheme or the time-division scheme, the voltage-transmittance characteristics are represented by two curves (two VT curves). For example, when an input signal with a gray scale value associated with a voltage denoted by reference character V1 in FIG. 15 is provided, for a pixel where bright display is to be performed (hereinafter, referred to as a “bright pixel”), the source voltage is determined based on a VT curve denoted by reference character 83 so as to obtain transmittance denoted by reference character T1, and for a pixel where dark display is to be performed (hereinafter, referred to as a “dark pixel”), the source voltage is determined based on a VT curve denoted by reference character 84 so as to obtain transmittance denoted by reference character T2. A correspondence relationship between an input gray scale (a gray scale value represented by an input signal) and an output gray scale (a gray scale value directly used to determine a source voltage) is, for example, represented by a curve denoted by reference character 85 in FIG. 16 for the bright pixel, and represented by a curve denoted by reference character 86 in FIG. 16 for the dark pixel. Note that data representing such correspondence relationships is typically stored in a look-up table in which input gray scales are associated with output gray scales for the bright pixel and the dark pixel (hereinafter, referred to as the “gray scale value conversion look-up table”). Although here description is made assuming that “bright” and “dark” are arranged in the unit of pixels, the same also applies to the case in which “bright” and “dark” are arranged in the unit of picture elements.
FIG. 17 is a diagram schematically showing an example of the gray scale value conversion look-up table. In FIG. 17, for example, a row denoted by reference character 87 refers to that “when the input gray scale is 192, the output gray scale is 240 for the bright pixel and the output gray scale is 32 for the dark pixel”. Therefore, in the case in which a liquid crystal display device adopting the time-division scheme uses the gray scale value conversion look-up table shown in FIG. 17, when there is included data with an input gray scale of 192, the output gray scale of a pixel associated with the data changes in the manner shown in, for example, FIG. 18. Note that an invention of a liquid crystal display device that adopts a time-division scheme such as that described above to improve viewing angle characteristics is disclosed in, for example, Japanese Patent Application Laid-Open No. 7-121144.